Voiceband modems for providing digital communications between computers via twisted pair telephone lines are well known. Voiceband modems are commonly used to provide Internet access by facilitating digital communications between personal computers and Internet Service Providers (ISPs).
Because voiceband modems operate within the limited bandwidth of the Public Switched Telephone Network (PSTN), i.e., 0 Hz to 3,400 Hz they are only capable of providing data rates up to 56 kbps.
However, due to the increasingly large quantity of digital data being communication via twisted pair telephone lines, the maximum bit rate associated with voiceband modems is frequently considered inadequate. The comparatively slow speed of voiceband modems is a severe limitation when transferring large binary files such as images, film clips, audio, large data files and the like. At 56 kbps, such files may require an undesirably long amount of time to transfer between computers when utilizing voiceband modems. Further, many applications, such as those involving real-time video, are not possible.
In an attempt to mitigate the data transfer rate limitation associated with such contemporary voiceband modems, integrated services data network (ISDN) modems have been developed. Not only do such ISDN modems increase the data rate to approximately 112 Kbps in some instances, but ISDN also facilitates simultaneous use of multiple communications devices simultaneously. For example, an ISDN system may be configured so as to allow the simultaneous transmission of data from a computer and voice from a digital telephone. The use of ISDN necessitates the installation of an ISDN compatible switch by the telephone company.
The increased data rate of ISDN modems with respect to voiceband modems is due to the use of a substantially larger frequency spectrum, i.e., 0 Hz to 80 kHz, and the corresponding higher signaling rates which can be used.
The increasing popularity of such communication services as video on demand, video teleconferencing and high speed Internet access has further increased the need for higher data rates over twisted pair telephone lines. Even the comparatively high speed associated with ISDN is not adequate for providing such services as pay per view and real-time videoconferencing, which require data rates of at least 1.5 Mbps.
Digital subscriber line (DSL) provides a way of facilitating digital communications over twisted pair telephone lines at data rates in excess of 1.5 Mbps, so as to facilitate such desirable services as video on demand, video teleconferencing, high speed Internet access and the like.
It is worthwhile to note that, although fiber optic cable will provide data rates in excess of those possible utilizing DSL on twisted pairs, the installation of fiber optic cable to customer premises is costly and is expected to take more than a decade. Therefore, it is necessary to leverage existing twisted pair copper wiring. It should be noted that this alternative is particularly attractive to telephone companies, since their existing infrastructure provides the telephone companies with a distinct time-to-market advantage in the highly competitive communications business.
There are currently several different versions of DSL available. These include basic digital subscriber line (DSL), high data rate digital subscriber line (HDSL), single line digital subscriber line (SDSL), asymmetric digital subscriber line (ADSL) and very high bit rate digital subscriber line (VDSL).
Basic DSL provides a data rate of 160 kbps simultaneously in both directions over a single twisted pair of telephone lines for distances of up to approximately 18,000 feet.
HDSL is an extension of basic DSL and provides an improved method for transmitting T1/E1 signals. T1 is used primarily in North America and Japan and facilitates the simultaneous transmission of 24 digitized voice channels. E1 is used in most of the rest of the world and supports up to 30 simultaneous digitized voice channels.
HDSL uses an advanced modulation technique to facilitate a data rate of 1.544 Mbps over a twisted pair telephone line for a distance of up to approximately 12,000 feet. HDSL requires two twisted pairs of telephone lines, each twisted pair operating at 768 Kbps.
SDSL is a single line version of HDSL. In SDSL, T1/E1 signals are communicated over a single twisted pair. SDSL is suitable for such applications as servers and power LANs, which require symmetric data communications, wherein equal data rates in both the upstream and downstream directions are provided. SDSL is also suitable for such services as private line and frame relay.
ADSL is well suited for video on demand, home shopping, Internet access and remote LAN access, wherein the downstream data rate is comparatively high with respect to the upstream data rate. As mentioned above, the communication of video, such as MPEG movies, can require data rates in excess of 1.5 Mbps. However, this high bit rate is in the downstream direction only. The upstream control signals, which may be from simulated VCR controls, may require as little as 16 Kbps. It has been found that a ten to one ratio of downstream to upstream data rates is suitable for many such data communications applications.
VDSL, like ADSL, utilizes asymmetric data communications. However, VDSL operates at much higher data rates, which are facilitated by requiring shorter transmission distances via the twisted pair telephone lines. Further, a symmetric version of VDSL may be utilized in multimedia applications requiring similar data rates in both directions.
The various different types of DSL may be referred to collectively as either DSL or xDSL. DSL utilizes an advanced modulation scheme known as quadrature amplitude modulation (QAM), wherein a combination of amplitude and phase modulation is used to encode digital information for transmission over various media, including twisted pair copper telephone lines. Quadrature amplitude modulation (QAM) is based upon suppressed carrier amplitude modulation of two quadrature carriers, i.e., two carriers having a phase relationship of 90 degrees with respect to one another. Quadrature amplitude modulation (QAM) is an extension of multiphase shift keying modulation schemes, such as quadrature phase shift keying (QPSK). The primary difference between quadrature amplitude modulation (QAM) and quadrature phase shift keying (QPSK) is the lack of a constant envelope in quadrature amplitude modulation (QAM) versus the presence of such a constant envelope in phase-shift keying techniques.
Although quadrature amplitude modulated signals are theoretically allowed to have any number of discrete digital signal levels which the physical media will accommodate, common implementations of QAM systems have constellation sizes defined by powers of two, such as QAM-4, QAM-8, QAM-16, QAM-32, QAM-64, QAM-128, and QAM-256, wherein the number indicates how many discrete digital levels are utilized.
Thus, it will be appreciated that the use of quadrature amplitude modulation (QAM) facilitates the simultaneous transmission of a larger number of bits per symbol interval, e.g., up to 8 bits with QAM-256, so as to provide substantially enhanced bit rates. Each such simultaneous transmission of a plurality of bits is accomplished by encoding the bits into a symbol. Of course, the use of symbols which contain a larger number of bits requires higher signal to noise ratios (SNR) for adequate resolution. Although quadrature amplitude modulation (QAM) does provide a substantial increase in bit rate, as compared with earlier modulation schemes, such as those which are utilized in ISDN modems, it is still desirable to optimize the bit rate provided by quadrature amplitude modulation (QAM), so as to provide digital communication at the highest possible speed while maintaining the desired quality of service.
In implementing quadrature amplitude modulation (QAM), parameters such as symbol rate, center frequency and constellation size must be selected in a manner which tends to optimize the data rate and/or transmission SNR margin. The maximum symbol rate is uniquely determined by the communication bandwidth of the transmission medium. Thus, the maximum symbol rate depends upon the type of media, e.g., twisted pair copper telephone lines, coaxial cable, fiber optic cable, etc. utilized and also depends upon the amount of noise present in the environment of the transmission medium.
As those skilled in the art will appreciate, the useable bandwidth of any transmission medium is determined to a substantial degree by the amount of noise which is undesirably introduced into the transmission medium. For example, in a transmission medium having a nominal bandwidth of 300 kHz to 10 MHz, the undesirable ingress of environmental noise between 8.5 MHz and 10 MHz may limit the useable bandwidth to 300 kHz to 8.5 MHz.
The center frequency, like the symbol rate or bandwidth, depends upon the transmission medium and the quantity and nature of environmental noise.
The constellation size used according to quadrature amplitude modulation (QAM) is dependent upon the bandwidth, center frequency and signal to noise ratio (SNR). The signal to noise ratio (SNR) is dependent upon both the type of transmission medium and the presence of environmental noise.
Since the bit rate depends upon the symbol rate, center frequency and constellation size, it is desirable to optimize symbol rate, center frequency and constellation size in order to provide digital communication at an enhanced bit rate for use in such applications as DSL.